In reply to TheGeneralist:
On the flat your bike weight will make little to no difference to the speed you roll at for any particular power output.
It will make a difference to your ability to accelerate.
Did the pace pick up steadily, you initially kept pace but then fell out the back or was there a constant stream of 'attacks' or accelerations, one of which finally snapped the elastic?
Also, was it truly flat or rolling?
Unless you are really (really) skinny then ignore the bike weight and just get stronger / lighter, but don't forget to train for explosive accelerations - which requires something slightly different to the constant power of a hill climb.
ps. If you have the power to weight ratio, plus the acceleration ability, and you've got the stamina from plenty of base miles and you still get dropped from the group as the pace picks up then I would serious reevaluate tactics. Are you really being accurate in how much effort you are putting in at the front early on? Are you fueling right?
Winners usually draft quite a bit